This invention relates to automatically operated extrusion systems, their methods of operation and their control. More particularly, this invention relates to extrusion methods, systems and controls that safely operate without a human operator in attendance, shutting down automatically in the event of an irremediable malfunction.
Extrusion systems are known that correct for dimensional variances in the extrudate. For example, my U.S. Pat. No. 4,209,476, issued Jun. 24, 1980, incorporated herein by reference, describes, among others, a tube extrusion system in which a gear pump accurately meters the throughput of melt, a speed sensor measures the speed of movement of a tubular extrudate and a gauge measures the outside diameter of the tubular extrudate. Inside diameter is calculated. Departure from the desired inside or outside diameter causes a controller to vary a dimension-affecting parameter such as the gear pump speed, the puller speed, the air pressure inside the tube or the vacuum outside the tube. Sold by Harrel, Incorporated, the assignee of the present invention, tinder the trademark TUBETROL(copyright), such a system works well for its intended purpose. If left unattended, it does not, however, shut down the line when there occurs either a severe malfunction of the system or a dimensional anomaly that cannot be remedied.
It would be desirable to have an extrusion system operable overnight, with no operator in attendance. In that case, just an alarm in the form of a siren or flashing light will not suffice in the event of a malfunction. No one is nearby to respond. Continued operation of the line could be disastrous.
For the purpose of detecting whether an extrusion system is operating correctly, it is not enough just to measure extrudate dimensions at a point along the extrudate""s path of movement. An extrudate that has ceased movement (or is not moving at the appropriate speed) may have the desired dimensions. A gauge on a point on its path of movement will detect that an appropriately dimensioned extrudate is present where it should be. The gauge will not detect the lack of appropriate movement. Meanwhile, still hot plastic is emerging from the die upstream of the gauge. A system in this condition, left unattended overnight will present its operator with a large cleanup task, a waste of material and perhaps even equipment damage upon discovery.
Similarly, although controls like the commercially available Harrel TUBETROL(copyright) are excellent in their day-to-day control of an extrusion line, if left unattended overnight, and if for some unforeseen reason, out-of-dimension product cannot be brought back into spec, the operator again is faced with an unpleasant surprise on return to the line. An entire shift""s worth of unacceptable product may have been manufactured.
A malfunctioning cutter that has ceased cutting extrudate regularly into prescribed lengths is another hazard that can be disastrous to the unattended extrusion line. If the extrudate is not severed or permitted to move away from the line, the likely result, again, can be a shift""s worth of damaged product.
In accordance with the present invention, an extrusion system is provided in which all power to the extrusion system is supplied via a switchable device or devices under the control of an extrusion controller coupled to a motion detector. In the event that the extrudate stops moving or does not move at the appropriate speed, the controller cuts off all power to the system. In other words, if the motion detector does not detect appropriate movement, either because the extrudate has stopped moving or because the extrudate is moving at the wrong speed, the extrusion system is shut down entirely.
In addition to the motion detector, the controller of the present system can receive an input from a dimension-measuring gauge. A corrective action undertaken by the controller to vary a dimension affecting parameter of the system should result in the gauge sensing and providing to the controller as an input a corrected dimensional measurement. If the measured dimension does not so-respond, the controller then causes interruption of power to the system by the switchable power device. Alternatively, or in addition to the controller""s sensing of a failure to respond to an adjustment of a dimension affecting parameter, the controller can be made to cause power interruption to this system in response to the detection of a dimensional anolomy so severe as to be beyond the control of the controller.
The motion detector coupled to the controller ordinarily includes a roller or rollers engaging the extrudate being formed to confirm that it is moving linearly along its path of movement at the correct speed. A motion detector in the form of a photodetector at the downstream side of an extrudate cutter can serve as the linear movement detecting motion detector, or can be used in addition to the linear movement detecting motion detector. As an extrudate is continually cut into lengths, the severed length typically drops or moves laterally out of the path of linear movement of the extrudate. The photodetector detects either the movement of the severed section out of its field of vision, or the movement of the severed extrudate section past its field of vision, depending upon the positioning of the photodetector. An output from the photodetector is input to the extrusion system controller. Programming of the controller looks for this input and resets a timer. If within a prescribed time no severance is indicated, the controller causes the interruption of electrical power to the system. A controller suitable to run this and other programs described here is the Harrel DIGIPANEL(copyright) Controller available from Harrel, Incorporated, East Norwalk, Conn.
In the case of the extrusion of a tubular extrudate, a TUBETROL(copyright) type of control program such as is described in my earlier U.S. Pat. No. 4,209,476 is used. A metering pump in the form of a gear pump delivers extrudate to the die of the extrusion system. This pump very accurately controls the volume of melt delivered to the die. As the extrudate emerges as a tube, its outside diameter is measured and its rate of linear movement is measured. From the speed of the metering pump, the volume of melt being delivered is known and the thickness of the tube wall is calculated. Using this technique, power to the extrusion system can be interrupted in the event that there occurs any one of a failure of the extrudate to move linearly along its path at the appropriate speed, an irremediable outside diameter error, or an irremediable tube wall thickness error.